The present invention relates to a polyaminopolyamide-epichlorohydrin (PAE) resin and its use in papermaking. It also relates to a process of activating wet strength resins through treatment with a base improving the wet strength performance of the PAE resin.
Wet strength is useful in a wide variety of paper grades, including tissue, toweling, packaging and publication and laminating grades. Wet strength is useful in a wide variety of applications, some examples of which are facial tissue, kitchen towel, milk and juice cartons, paper bags and recycled liner board for corrugated containers.
Dry strength is another important paper property, particularly in light of the trend for paper manufacturers to use recycled fibers in paper in order to achieve lower costs. Wet strength resins can also provide increased dry strength to paper.
Commercially, polyaminopolyamide-epichlorohydrin (PAE) resins are used extensively as wet-strength additives. Commercial PAE resins can contain 1-10% (dry basis) of 1,3-dichloropropanol (1,3-DCP) and 3-chloropropanediol (also known as 3-CPD, 3-MCPD or CPD), which are by-products of the epichlorohydrin (epi). These epi by-products are carcinogenic and are closely regulated. For example, under the European regulation on Registration, Evaluation, Authorization and Restriction of Chemicals (REACH), 1,3-DCP is considered to be a Substance of Very High Concern (SVHC). As such, wet-strength resins that contain more than 1000 ppm of 1,3-DCP cannot be manufactured or imported into the European Union. Production of wet-strength resins with reduced levels of epi by-products has been the subject of much investigation. Environmental pressures to produce wet-strength resins with lower levels of absorbable organic halogen (AOX) species have been increasing. “AOX” refers to the absorbable organic halogen content of the wet strength resin which can be determined by means of adsorption of the halogen onto carbon. Accordingly, AOX includes epichlorohydrin (epi) and epi by-products (1,3-dichloropropanol, 2,3-dichloropropanol and 3-chloropropanediol) as well as organic halogen bound to the polymer backbone (polymer-bound AOX). Polymer-bound AOX includes aminochlorohydrin functionality and polymer-bound CPD (CPD-ester), which is a by-product of the reaction of epi with an acid end-group on the polyaminopolyamide.
Several ways of reducing the quantities of epihalohydrin by-products and polymer-bound AOX have been devised. Reduction in the quantity of epihalohydrin used in the synthetic step is an alternative. Control over the manufacturing process is another option yielding compositions of reduced concentration of hydrolysis products. Post-synthesis treatments are also known. It is also known that epichlorohydrin and epichlorohydrin hydrolyzates can be reacted with bases to form chloride ion and polyhydric alcohols. Bases can be used during the synthetic step to reduce organo chlorine contents of wet strength composition to moderate levels. U.S. Pat. No. 5,019,606 and U.S. Pat. No. 4,929,309 teach reacting wet strength compositions with an organic or inorganic base and then stabilizing against gelation with acid.
Still further, U.S. Pat. No. 5,972,691 discloses the treatment of wet strength compositions with an inorganic base after the synthesis step (i.e., after the polymerization reaction to form the resin) has been completed and the resin has been stabilized at low pH, to reduce the organo halogen content of wet strength compositions (e.g., chlorinated by-products) to moderate levels (e.g., about 0.5% based on the weight of the composition). The composition so formed can then be treated with microorganisms or enzymes to economically produce wet strength compositions with very low levels of free epihalohydrins and epihalohydrin by-products.
U.S. Pat. No. 7,175,740, assigned to Hercules Incorporated, teaches either an acid treatment, base treatment or an enzyme treatment to destroy polymer-bound CPD (e.g., CPD-ester). U.S. Pat. No. 7,081,512 teaches a base treatment. However, both these base treatments are followed by acid addition to produce a gelation stable resin.
Moreover, U.S. Pat. No. 6,429,267, assigned to Hercules Incorporated, which is herein incorporated by reference in its entirety, discloses amongst other features, a process for reducing the AOX content of a starting or initial water-soluble wet-strength resin comprising azetidinium ions and tertiary aminohalohydrin, which includes treating a starting or initial resin having a Gardner-Holdt viscosity of “C” or higher at 20% solids, in aqueous solution with base to form a treated resin, wherein at least about 20 mole % of the tertiary aminohalohydrin present in the starting resin is converted into epoxide and the level of azetidinium ion is substantially unchanged, and the effectiveness of the treated resin in imparting wet strength is at least about as great as that of the starting wet-strength resin.
U.S. Pat. No. 8,399,544, assigned to Hercules Incorporated, which is herein incorporated by reference in its entirety, discloses adhesive compositions which use a high solids, low viscosity PAE resin.
Each of the foregoing approaches has provided various results, and there has been a continuing need for further improvement in performance and AOX in polyamine-epihalohydrin resin-based compositions, such as wet strength agents, dry strength agents, creping adhesives, and wood product adhesives. There also remains a need for polyamine-epihalohydrin resins and polyamine-epihalohydrin resin compositions having high solids content and providing higher performance (e.g., higher wet/dry strength) and very low AOX, as well as paper products and wood product comprising such resins.
Higher wet strength and higher wet/dry strength is an unmet need for industry. For example, achieving higher wet/dry strength would allow paper manufacturers to provide products (e.g., kitchen towel) with higher wet strength to meet consumers' demands. Additionally, paper manufacturers can further modify the pulp furnish (e.g., adding more hard wood pulp, such as eucalyptus pulp) to lower manufacturing costs while increasing the softness of their products.
The present invention, relates to a process of activating a PAE resin to a target molecular weight, measured via viscosity, and subsequent dilution to a stable form. The activated or treated resin exhibits significantly improved performance over the starting resin and exhibits greater performance than standard PAE resins at addition levels greater than 1%.
Therefore, in addition to the benefit of very low AOX, it also can achieve the unmet need to achieve higher wet strength and higher wet/dry strength than achievable with current technology.
Additionally, the present process can achieve the unmet need of allowing PAE resin manufacturers to ship a REACH compliant, gelation stable, high solids resin, saving shipping and environmental costs while still having a resin that is easily pumped at high solids due to its relatively low viscosity for a high solids resin.
All of the references noted above are herein incorporated by reference in their entirety.